Mercury once had a graphite crust floating on a sea of magma

To brighten up the innermost planet, you might need an eraser. A mystery material on Mercury’s surface seems to be graphite, the same substance found in pencil lead – and the entire planet may have once been encrusted in it.

Squint a bit and Mercury looks quite a lot like the moon. Both are rocky bodies in the inner solar system with little atmosphere or geological activity.

But closer scrutiny reveals that Mercury’s surface reflects much less sunlight than the moon, for reasons we couldn’t explain. Iron and titanium might be to blame, but NASA’s now-defunct Messenger probe didn’t find enough of either element on the planet to account for the difference.

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Now, thanks to a clever analysis of Messenger data acquired right above the dimmest parts of Mercury’s surface, we may have an answer: infrared spectra show the dark stuff is carbon, in the form of graphite. What’s more, the number of neutrons sparked by cosmic rays hitting the surface is higher than it would be from other possible minerals.

“The only hypothesised darkening agent that works with these two data sets is carbon,” says Patrick Peplowski of the Johns Hopkins University Applied Physics Laboratory in Maryland, whose team published the finding.

The graphite may date back to the earliest days of Mercury, when a magma ocean covered the planet. Assuming the planet had the same chemistry as it does today, nearly every mineral that formed in the ocean would have sunk to the bottom. “There’s only one mineral that would float, and it’s graphite,” Peplowski says.

As it cooled, Mercury would have been covered in a shell of graphite perhaps 1 kilometre thick. Later lava flows would have buried this darker layer.

That would mean that today, the darkest material on Mercury should show up in craters where the original surface has been gouged out – exactly what Peplowski’s team found.

“I think it’s pretty convincing, at least for these spots of very dark material,” says Simone Marchi, at the Southwest Research Institute in Boulder, Colorado.